With the advance of laser processing machines, magneto-optical devices utilizing the interaction of light and magnetism have recently become of much interest. One of these devices is an isolator, which functions to inhibit the phenomenon that if the light oscillated from a laser source is reflected by the optical system in its path and is returned to the light source, then it disturbs the light oscillated from the laser source, to cause an unstable oscillation state. Accordingly, on use, the optical isolator is arranged between a laser source and an optical member so as to take advantage of the function.
The optical isolator comprises three parts, a Faraday rotator, a polarizer arranged on the light-input side of the Faraday rotator, and an analyzer arranged on the light-output side of the Faraday rotator. The optical isolator utilizes the nature, commonly known as the Faraday effect, that when light enters the Faraday rotator under the condition where a magnetic field is applied to the Faraday rotator in a direction parallel to the light traveling direction, the plane of polarization is rotated in the Faraday rotator. Specifically, a component of the incident light having the same plane of polarisation as that of the polarizer is transmitted by the polarizer and enters the Faraday rotator. The light is rotated by +45 degrees relative to the light-traveling direction in the Faraday rotator, and then emerges therefrom.
By contrast, when the return light entering the Faraday rotator in an opposite direction to the incident direction first passes through the analyser, only a component of the light having the same plane of polarization as that of the analyzer is transmitted by the analyzer and enters the Faraday rotator. In the Faraday rotator, the plane of polarization of the return light is further rotated by +45 degrees in addition to the initial +45 degrees. Since the plane of polarization of the return light is at a right angle of +90 degrees with respect to the polarizer, the return light is not transmitted by the polarizer.
The Faraday rotation angle θ is represented by the following formula (A).θ=V*H*L  (A)In formula (A), V is a Verdet constant which is determined by the material of the Faraday rotator, H is a magnetic flux density, and L is the length of the Faraday rotator. For use as an optical isolator, L is determined so as to give θ=45 degrees.
It is important that the material to be used for the Faraday rotator of the optical isolator mentioned above have a significant Faraday effect and a high transmittance at the wavelength on use.
Also, if a polarized component different from the incident light is generated in the output light, this different polarized component is transmitted by the polarizer, indicating insufficient blockage of the return light.
For the evaluation of the generation of a different polarized component, polarized light of 0 to 90 degrees enters a material used as the Faraday rotator, output light is transmitted by the polarizer into a photodetector, and the intensity of light is measured by the photodetector. From the maximum intensity (Imax) and minimum intensity (Imin), an extinction ratio (S) is computed according to the following equation.S=10 log(Imin/Imax)  (unit: dB)While higher values of extinction ratio are important, an extinction ratio of at least 30 dB is generally required.
Recently, JP-A 2010-285299 (Patent Document 1) discloses a single crystal oxide of (TbxRe1-x)2O3 (wherein 0.4≦x≦1.0) and transparent oxide ceramics as the material having a high Verdet constant.
Also JP 4033451 (Patent Document 2) discloses a rare earth oxide represented by the general formula; R2O3 wherein R is a rare earth element, which is free of birefringence since its crystal structure is cubic. It is described that a sintered body having a high degree of transparency can be produced if pores and impurity segregates are completely removed.
Further, JP-A H05-330913 (Patent Document 3) describes that addition of sintering aids is effective for removing pores. JP 2638669 (Patent Document 4) discloses removal of pores by hot isostatic pressing and re-sintering. One preparation method involves adding one or more of the sintering aids disclosed in JP-A H05-330913 (Patent Document 5) or the like, mixing, compacting, calcining, sintering in vacuum, and HIP treating.
JP-A 2010-285299 (Patent Document 6) discloses a transparent oxide ceramic material of (TbxRe1-x)2O3 (wherein 0.4≦x≦1.0). Although this ceramic material basically has a cubic crystal structure, it sometimes exhibits faint birefringence because a sintering aid incorporated therein can react with the main component to form a phase different from the cubic crystal that precipitates within crystal grains or at grain boundaries. This may invite a lowering of extinction ratio.
Since the precipitates are of submicron size, laser light irradiated to the ceramic material is scattered there. Due to scattering, the insertion loss may be reduced.
Also when ceramic materials are sintered, the composition of the main component (TbxRe1-x)2O3 and the concentration of sintering aid vary between the interior and the outer periphery of the ceramic material due to segregation, resulting in variations of extinction ratio and insertion loss within the ceramic surface.